1. Field of the Invention
This invention relates to variable capacitors and, more particularly, to variable capacitors with electrostatic capacitances made variable by changing the effective opposing or overlapping area of a stator electrode and a rotor electrode due to rotation of the rotor electrode relative to the stator electrode.
2. Description of the Related Art
A prior known variable capacitor 1 is shown in FIGS. 17 and 18. FIG. 17 shows a plan view whereas FIG. 18 is a cross-sectional view along line XVIII--XVIII of FIG. 17.
The variable capacitor 1 includes a stator 3 made of resin with a stator terminal 2 being insert-molded therein. The stator terminal 2 is integrally formed with a stator electrode 4 which is exposed to the upper surface of the stator 3. A rotor 5 made of a dielectric material is disposed on the upper surface of the stator 3 while a rotor electrode 6 is formed on the upper surface of rotor 5 in a way such that it opposes stator electrode 4 with rotor 5 being positioned therebetween.
A shaft 7 made of metal is rotatably mounted by penetrating the stator 3, with the above-mentioned rotor 5 being coupled to the shaft 7 in such a way that the rotor 5 is integrally rotatable with this shaft 7. The shaft 7 forms a head 8 on its upper end section. The head 8 includes a driver groove 9 formed in the upper surface of this head 8. In addition, the shaft 7 is electrically connected to the rotor electrode 6 via a projection 10.
A disk-like spring 11 is engaged with the shaft 7 on the lower surface side of the stator 3. The lower end section of the shaft 7 is locked or caulked whereby disk spring 11 is in the state that it creates an elastic force which acts to compress rotor 5 toward stator 3 through shaft 7. The disk spring 11 also permits integral formation of a rotor terminal 12.
In the variable capacitor 1 thus arranged, the electrostatic capacitance as formed vis-a-vis the facing arrangement of the stator electrode 4 and rotor electrode 6, the leads to which are taken out between the stator terminal 22 and rotor terminal 12. Also, the electrostatic capacitance is variable by rotating the rotor 5 via the shaft 7 so as to change the effective opposing or overlapping area of the stator electrode 4 and the rotor electrode 6.
On the other hand, FIGS. 19 and 20 show another variable capacitor 21 in accordance with the second prior art. FIG. 19 depicts a plan view whereas FIG. 20 is a cross-sectional view along line XX--XX of FIG. 19.
The variable capacitor 21 includes a casing 23 made of resin with a stator terminal 22 being insertion-molded therein. The casing 23 is provided with a recess section 24 in which a stator 25 made of a dielectric material is statically disposed with a rotor 26 made of metal being rotatably housed thereon.
A stator electrode 27 is formed on the lower surface of the stator 25. The aforesaid stator terminal 22 causes a contact section 28 to be integrally formed for being elastically contacted with the stator electrode 27. On the other hand, a rotor electrode 29 is formed on the lower surface of the rotor 26 such that it opposes the stator electrode 27 with the stator 25 therebetween. A driver groove 30 is formed in the upper surface of the rotor 26.
The upper opening of the recess section 24 is covered by a cover 32 made of metal, except for an adjustment hole 31 which exposes the driver groove 30 to the outside. The cover 32 integrally forms a spring contact piece 33 which elastically biases the upper surface of the rotor 26 away from a rotor terminal 34. After inserting the stator 25, the rotor 26 and the cover 32 into the recess section 24, the upper end edge 35 of the casing 23 is heated and deformed to change from the state designated by imaginary or phantom line to the state as indicated by the solid line in FIG. 20, whereby the cover 32 is secured to the casing 23 while at the same time allowing the spring contact piece 33 to produce elastic force which acts to compress the rotor 26 against the stator 25.
In the variable capacitor 21 structured as described above, the electrostatic capacitance as formed vis-a-vis the facing arrangement of the stator electrode 27 and the rotor electrode 29, and is measurable between the stator terminal 22 and the rotor terminal 34. The electrostatic capacitance is variable by changing the effective opposing areas of the stator electrode 27 and the rotor electrode 29 due to rotation of the rotor 26.
However, the first and second prior art devices stated above encounter problems, such as follow.
First, in the variable capacitor 1 in accordance with the first prior art embodiment shown in FIGS. 17 and 18, the disk spring 11 is used to cause the stator 3 and the rotor 5 to be in close contact with each other. However, minute size variations can occur at respective portions due to heating during a solder reflow process, which is applied when this variable capacitor 1 is assembled on a printed circuit board. These size variations might lead to a decrease in spring pressure as applied by the disk spring 11, which would result in the risk of loosening the aforesaid close contact state, which in turn serves to decrease the torque for use in rotating the shaft 7 and/or let the electrostatic capacitance be rendered unstable due to drift in the setting.
In addition, while a hand tool such as a screwdriver is inserted into the driver groove 9 when an attempt is made to adjust the electrostatic capacitance, since the head 8 of shaft 7 in which such driver groove 9 is formed is placed so as to project from the remaining portion of this variable capacitor 1, it is rather difficult to reliably insert the screwdriver or the like into driver groove 9. It also remains difficult to eliminate the risk that the screwdriver or the like from spinning off from driver groove 9 after insertion thereof. For these reasons the workability of capacitance adjustment remains relatively low.
Further, during soldering using reflow techniques or soldering irons, flux can move or drift along the shaft 7 or splash and scatter therearound, thereby invading the sliding faces of the stator 3 and the rotor 5. As a result, the electrostatic capacitance stability and Q characteristics can decrease.
Furthermore, since the stator electrode 4 is embedded in the stator 3 made of a resin material that is easily deformable, both thermally and mechanically, a contact with the rotor 5 can become unstable, resulting in the electrostatic capacitance being unstable accordingly.
Next, in the variable capacitor 21 in accordance with the second prior art device shown in FIGS. 19 and 20, after the stator 25, the rotor 26 and the cover 32 are inserted into the recess section 24, the cover 32 is then secured to the casing 23 by causing the upper end edge 35 of the casing 23 to deform while heat is applied thereto.
However, apparatus for heating the casing 23 for such deformation is generally large-scaled in structure resulting in an increase in cost of the variable capacitor 21. In addition, while cooling must be carried out while retaining deformation of the upper end edge 35 of the casing 23, a relatively long time duration might be required for accomplishment of this cooling process.